Plastic lenses and glass lenses often perform the same function in optical systems, such as in cameras, microscopes, telescopes and opthalmic wear. The two main attributes that separate plastic lenses from glass lenses are cost and optical stability. Plastic lenses typically cost 1/100th the price of a similar glass lens. On the other hand, the stability of the refractive index of a glass lens with respect to temperature and humidity is typically 100 times better than that of a plastic lens.
The difference in cost is due largely to the difference in manufacturing processes that are required for the two materials and the relative temperatures that the materials must be formed at. Plastic lenses are typically produced at 230° C. using injection molding at cycle times that are 10 times faster than glass lenses that are largely produced by grinding and polishing or compression molding at 625° C. Grinding and polishing are labor intensive while the high temperatures that glass must be formed at requires expensive mold materials and extensive maintenance costs.
In contrast, the difference in optical stability between plastic and glass is due to differences in their basic material properties. This difference in optical stability results in substantially more variation in focus and image quality in articles such as cameras when plastic lenses are used in place of glass. What is desired, and a remaining challenge in the art, is a material with the optical stability of glass that processes like a plastic. While optical plastic materials such as cyclic olefins greatly improve the refractive index stability with respect to humidity, improving the refractive index stability with respect to temperature has remained an opportunity A study on the competing fundamental material characteristics that determine the sign and the magnitude of the dn/dT of glasses is available, for instance, by Lucien Prod'homme, “A new approach to the thermal change in the refractive index of glasses,” Physics and Chemistry of Glasses, Vol. 1, No. 4, August. The two competing effects that determine the dn/dT in glasses are the density change which produces a negative dn/dT and the electronic polarizability which produces a positive dn/dT. The net dn/dT in a glass material depends on which effect dominates. In optical plastics however, there is not an electronic polarizability so that all unfilled materials have negative dn/dT values. None the less, the article by Prod'homme does identify the possibility of using glass-like fillers with positive dn/dT values to substantially alter the dn/dT of a filled plastic composite material.
Nanoparticulate fillers have been used to modify the index of refraction of optical plastics. By using a filler small enough that it is well below the wavelength of visible light (400–700 nm), the filler will not scatter the light and the filled plastic can retain its transparency. WIPO Patent WO97/10527 describes the use of nanoparticles to increase the refractive index of plastics for opthalmic applications. In addition, technical references that describe the addition of nanoparticles to increase the refractive index of plastics include: C. Becker, P Mueller, H. Schmidt; “Optical and Thermomechanical Investigations on Thermoplastic Nanocomposites with Surface-Modified Silica Nanoparticles,” SPIE Proceedings Vol. 3469, pp. 88–98, July 1998; and, B. Braune, P. Mueller, H. Schmidt; “Tantalum Oxide Nanomers for Optical Applications,” SPIE Proceedings Vol. 3469, pp. 124–132, July 1998. While these references disclose the use of nanoparticles to modify refractive index of optical plastics they do not discuss the issue of refractive index stability with respect to temperature which requires a different set of characteristics in the nanoparticle.
U.S. Pat. No. 6,020,419 issued to M. Bock, et al., discloses the use of nanoparticulate fillers in a resin based coating for improved scratch resistance. U.S. Pat. No. 5,726,247 issued to M. Michalczyk, et al , also describes a protective coating that incorporates inorganic nanoparticles into a fluoropolymer. While scratch resistance is important in plastic optics, the nanoparticles that would be suitable for scratch resistance would be very different from those with the specific properties needed to improve refractive index stability with respect to temperature.
U.S. Pat. No. 3,915,924 issued to J. H. Wright describes a nanoparticulate filled clear material for filling voids. U.S. Pat. No. 5,910,522 issued to H Schmidt, et al., describes an adhesive for optical elements that includes nanoscale inorganic particles to reduce thermal expansion and improved structural properties at elevated temperatures. While the inventions described in these patents represents some progress in the art, none of them address specific optical properties of the modified plastic material particularly as these properties relate to temperature sensitivity.
WIPO Patent WO9961383a1 discloses a method for producing multilayered optical systems that uses at least one layer that contains nanoparticulate fillers to form a layer with a different refractive index than the substrate to create an interference filter or an antireflection layer. Obviously, this patent is addressing another form of modification of the index of refraction and such is not concerned with the stability of the index of refraction with respect to temperature.
Skilled artisans will appreciate that a wide variety of materials are available in nanometer particle sizes that are well below the wavelength of visible light. Representative materials may be acquired from companies such as Nanophase Technologies Corporation and Nanomaterials Research Corporation. By selecting nanoparticle materials based on properties other than index of refraction, our experience indicates that it is now possible to modify other optical properties of plastics.
While there have been several attempts to modify properties of plastics using nanoparticles, none of these attempts have proven successful in producing optical plastic articles with temperature stable optical properties while retaining important processing characteristics.
Therefore, a need persists in the art for optical plastic articles, such as lenses, and a method of making same that have temperature stable optical properties.